The use of magnetic susceptibility measurements to delineate wetlands in KwaZulu-Natal, South Africa.

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Abstract

The aim of this research was to investigate the possibility of using soil magnetic susceptibility to differentiate wetland soil from non0wetland soil, thereby enabling the identification of a wetland boundary.
The initial methodology to test the viability of using magnetic susceptibility for wetland
delineation was carried out at nine sites in three areas of Kwazulu-Natal, South Africa. Changes in vegetation were used to identify the permanently, seasonally and temporarily wet zones at each site to provide a quick indication of the various boundaries. At least one transect was studied at every site,
extending from the outer edge of the temporary zone to the water's edge. Magnetic susceptibility
readings, soil samples and/or elevations were taken in each zone. It was found that the magnetic
susceptibility readings could be used to differentiate between the various wetland zones but
boundaries were not identified due to the reconnaissance nature of the work, a specific protocol had yet to be discovered. This methodology was a reconnaissance phase to assess the potential of using soil magnetic susceptibility.
A second methodology was designed specifically to identify a wetland boundary. The wetland zones
were identified using vegetation indicators from the South African field procedure for delineating
wetlands. Magnetic susceptibility readings were done on transects perpendicular to the suspected
boundary and a critical value was identified. A magnetic susceptibility boundary was marked according to the critical value and verified by taking readings along transects parallel to it on both
sides. The boundary was then identified using South African field procedure according to soil indicates. The soil data showed a similar boundary to the magnetic susceptibility boundary but
indicated a 'boundary zone' of approximately l2m in width rather than a specific line.
A final methodology was planned to improve the resolution of the magnetic susceptibility boundary.
A grid was laid out over a strip through the wetland including the boundary area on both sides. This
was to provide accurately spaced points at which to take magnetic susceptibility readings and
elevations. The critical value, a value that separates wetland from non-wetland soil, was identified
and verified using soil indicators. The field procedure was more difficult to carry out than using the
magnetic susceptibility sensor, yet both methods identified the same boundary with a resolution of
about 3m. This initial study demonstrates the potential for using magnetic susceptibility for wetland
delineation. Although the results at the final site proved the method to be successful, it was not
suitable for use at all sites and the results were often difficult to interpret. Limitations include factors
such as plinthic horizons close to the soil surface and shallow, rocky soils. Thus further research is
required before magnetic susceptibility can be used to delineate wetlands in KwaZulu-Natal, South
Africa.